Generate multi-files facade even if it's empty

It still contains useful information about it's parts,
that may be used while building stubs for multi-file group
containing only InlineOnly functions

compiler/testData/codegen/bytecodeListing/emptyMultifileFacade.txt

@@ -0,0 +1,9 @@
+@kotlin.Metadata
+@kotlin.jvm.internal.KotlinMultifileClass
+public final class test/Foo
+
+@kotlin.Metadata
+@kotlin.jvm.internal.KotlinMultifileClassPart
+synthetic final class test/Foo__EmptyMultifileFacadeKt {
+    private final static method privateOnly(): void
+}